Nowadays, a large number of dyes are known, and they are largely categorized into natural dyes and synthetic dyes. Examples of the synthetic dyes include aniline blue, fuchsine and methyl orange. Most of the synthetic dyes have an aromatic or heterocyclic ring, and they are classified as either ionic compounds (for example, all water-soluble dyes) or non-ionic compounds (for example, disperse dyes). In addition, in the case of the ionic dyes, they are categorized into anionic (negative ionic) dyes and cationic (positive ionic) dyes.
The cationic dyes comprise an organic cation having a positive charge delocalized over a conjugated bond and normally an inorganic anion. Also, the cationic dyes are generally dyes in which an amino group which may have a substituent is involved in resonance. Therefore, selection of the cationic dyes depends on the number and kinds of the anion being a counter ion. Examples of the counter anion include a chloride ion, a bromide ion, an iodide ion, a perchlorate ion, a tetrafluoroborate ion, a hexafluorophosphate ion, an alkyl or aryl sulfate ion, a tosilate ion, an acetate ion and an oxalate ion.
Generally, rhodamine, safranine and victoria blue, which are cationic dyes, contain a chloride ion or tosilate ion as a counter ion. However, these compounds have insufficient heat resistance.
An example is known, in which a chloride ion or an aryl sulfate ion is used as a counter anion of a triarylmethane dye to improve heat durability of the triarylmethane dye (for example, Patent Literature 1).
In Patent Literature 2, as a method of obtaining a color composition for color filters with excellent color characteristics, heat resistance, light resistance and solvent resistance, a salt-forming compound comprising a triarylmethane basic dye and a sulfonated organic compound having at least two sulfonic groups, is disclosed.
In Patent Literature 3, as a method of obtaining a coloring resin composition which has not only excellent light resistance but also excellent light resistance, a salt forming method has been reported, in which a salt is formed by using a sulfonated compound of a dye skeleton such as phthalocyanine or anthraquinone, which is the counter anion, in combination with a triarylmethane skeleton, which is the cation.
However, the salt-forming compounds containing a dye and a counter anion disclosed in Patent Literatures to 3 are insufficient in heat resistance. Accordingly, there has been a desire for a color material with increased heat resistance.
A polysiloxane dye is disclosed in Patent Literature 4, which is highly cross-linked by polysiloxane containing at least ten Si atoms. Due to its synthesis method, the polysiloxane dye disclosed in Patent Literature 4 is a mixture in which an unreacted compound having only one dye skeleton or dyes with different polymerization degrees are present. It is difficult to separate only a dye with a specific polymerization degree from the polysiloxane dye, so that there is a problem with the productivity of the polysiloxane dye. Since the polysiloxane dye contains a silanol group or alkoxysilyl group, a siloxane bond is formed between the polysiloxane dye(s) or between the polysiloxane dye and other component having a silanol group or alkoxysilyl group. As a result, there is a deterioration in the state of a solution or dispersion liquid comprising the polysiloxane dye, such as a change in solubility or an influence on dispersion stability, thus making it difficult to handle the solution or dispersion liquid. Such a reaction is likely to proceed particularly upon heating, therefore it is not suitable to use the polysiloxane dye under heating at high temperature. As will be described in Comparative Examples hereinafter, the above-mentioned polysiloxane dye is insufficient in heat resistance.